So what does that mean for cold sufferers right now? According to Barrett, it depends.

"If you're an adult who has tried echinacea in the past and been happy with it, this study does not say you should stop using it," he said. "On the other hand, it certainly doesn't tell you you should start."

The problem with the common cold is that it can be caused by any of over 200 different viruses. So it is ubiquitous (Americans alone come down with 1 billion colds each year) and resistant to the treatments we throw at it, with symptoms like congestion, sore throat and cough often lasting up to two weeks.

There are treatments that can make symptoms less severe, like acetaminophen for the headache and decongestants for the stuffy nose. But nothing -- from cold medications to vitamin C -- seems to speed recovery from the infection.

"There are no treatments out there that have been shown to reduce the duration of colds," Barrett said.

And though a decongestant or cough suppressant might bring some temporary reprieve, they can also have side effects like dizziness, drowsiness or insomnia. So many people opt for alternative remedies that are commonly seen as safer. One study found that from 2000 to 2006, Americans shelled out between $100 million and $200 million per year on echinacea alone.

For the current study, Barrett's team recruited 719 12- to 80-year-olds who had developed cold symptoms within the past 36 hours. They randomly assigned the patients to take nothing, use placebo tablets or use echinacea tablets; of the echinacea users, half knew they were taking the herb, while the half did not whether they were taking echinacea or the placebo.

The echinacea groups took a formulation made from the root of the plant, which is rich in constituents that, some studies suggest, would be most likely to battle cold symptoms. The patients took eight echinacea tablets on the first day of the study, then four a day over the next four days.

On average, the study found, people in both echinacea groups suffered cold symptoms for between six and seven days. The placebo and no-pill groups differed from them by only a matter of hours -- roughly a half-day.

Similarly, there was no clear difference when the researchers looked at the groups' self-rated symptom severity, which each patient had recorded twice a day while the cold lasted.

The researchers found no evidence that echinacea users suffered side effects, like headache, stomach upset or diarrhea, at a higher rate than the untreated groups.

That's in line with the low risk of side effects studies have generally found when echinacea is taken as directed -- though there is a small chance of allergic reaction.

As far as effectiveness, however, even the studies with positive results suggest echinacea has only modest benefits against cold symptoms.

"No large, well-done clinical trial has shown a large benefit," Barrett said. "It doesn't stop a cold in its tracks, that's for sure."

For people looking for non-drug ways to soften the blow of cold season, Barrett suggested tactics like getting plenty of fluids, getting enough rest and, yes, having some old-fashioned chicken soup.

Those things may not chase away a cold, but they can help you feel better while it's around, he said.

The current study was funded by the U.S. National Institutes of Health and the Robert Wood Johnson Foundation. Some of the co-researchers on the work are with Warwick, Australia-based MediHerb, maker of the echinacea product used in the study.